Increasing attention has been paid in recent years to the automation of trouble detection in telecommunication networks. Much of the attention has been devoted to the common circuitry of each node which is used by all communications through the node. Trouble analysis of the large number of functional circuits which are used to interconnect the nodes of a network, generally considered to be less critical than the common circuitry, is much less developed. Unlike much of the common circuitry, these circuits can rarely be monitored continuously in an efficient and cost effective manner.
In telecommunication systems, the functional circuits which interconnect the nodes of a network are called trunks and are customarily used as shared facilities which are made available to different customers of the network at different times. Normally, the trunks interconnecting any two nodes are grouped into trunk groups of varying sizes. When a connection between customers associated with different nodes of the network is to be set up, any available trunk from the group(s) interconnecting these nodes may be selected for the connection.
Occasionally, a marginal condition, such as excessive noise, occurs on a trunk; this condition is not detected by routine operational tests but is noticeable to the customer. When such a trunk is engaged in a call, the customer engaged in the connection detects the unacceptable condition, terminates the call after a very short time, and attempts to place the call again. Since such trunks are used for only a short time on each connection attempt, i.e., have short holding times, they become repeatedly available for customer use and are responsible for an inordinate number of unsuccessful call attempts. In telephone systems, these are sometimes referred to as "killer trunks" because of the excessive number of unsuccessful call attempts attributable to each such trunk.
The need to detect faulty trunks rapidly is particularly acute in private networks, for example, those dedicated to one large customer such as a corporation or government agency. Such networks typically have a relatively small number of heavily loaded trunks in a group. Because of such a deficiency, the individual using such a network is often subjected to the use of the same defective trunk several times before a successful call can be completed. Even when a customer complains about a bad connection, it is frequently difficult to associate the complaint with a specific trunk or trunk group.
Generally, many or all trunks of a group share some common circuits. All of the trunks in a group share a common connected node. When there is trouble in such common circuitry, or in the connected node, many calls distributed over a number of trunks are not completed successfully. It is important to be able to distinguish this type of trouble condition from trouble in circuitry individual to one trunk, since different remedial action is required.
Off-line facilities are commercially available for recording the number of times that a given trunk is seized (attempt counts) and for recording the percentage of time that a given trunk is being used (percent of usage or occupancy). From these records, it is possible to identify trunks which have an unusually low average holding time (ratio of usage to attempt count) and this will give an indication of defective trunks. However, these prior art systems are usually adapted to take measurements for only a small fraction of the trunks connected to a node since special equipment is required for every trunk that is monitored. The cost of monitoring all trunks with such equipment would be prohibitive. No good overall mechanism exists for identifying which trunks or trunk groups should be measured. Since meaningful measurements must usually cover several busy periods, a delay of weeks or months is encountered before the performance of every trunk connected to a node can be meaningfully measured with the use of such dedicated systems.
Furthermore, these prior art systems are restricted to use with trunks which provide direct circuit access to each trunk and cannot be used with trunks which carry multiplexed communications such as digital trunks or analog carrier trunks, unless such trunks are adapted to provide such access.